The most commonly used fluid level sensor is the variable resistor sensor utilizing a float to produce a resistance change in the variable resistor. As the float moves vertically with the fluid level, the electrical resistance of the sensor changes typically from 30 to 270 ohms. In most sensors, a sliding or moving contact attached to the float establishes a resistive circuit based upon the position of the contact with respect to a wirewound resistor or a thick film resistor printed on an insulating base or substrate.
Other approaches to fluid level detection include use of resistors with large temperature coefficients, known as thermistors, located at various vertical positions in the fluid reservoir. As electrical power is applied to the resistors, the devices immersed in the fluid remain cool while those that are exposed to air will increase in temperature and produce a change in overall resistance of the device. Extensive signal conditioning and temperature compensation circuitry is typically required with such a sensor to create a usable signal. Fluid compatibility and manufacturing costs limit widespread acceptance of this type of device.
A vertical sensor with a sliding contact has been used in some automotive applications. Typically a float provides a contact point with respect to a resistor. The resistor is usually a wire helix wound about an insulating mandrel.
Examples of prior art fluid level sensors are shown in the following patents: U.S. Pat. No. 4,827,769 to Riley et al., U.S. Pat. No. 4,779,460 to Cruickshank, U.S. Pat. No. 4,775,435 to Draeger, U.S. Pat. No. 4,702,107 to Guerrini et al., U.S. Pat. No. 4,637,254 to Dyben et al., U.S. Pat. No. 4,567,762 to Hoppert et al., U.S. Pat. No. 4,454,761 to Coulange, U.S. Pat. No. 4,052,901 to Bjork, U.S. Pat. No. 3,113,282 to Coleman, U.S. Pat. No. 3,106,693 to de Giers, U.S. Pat. No. 2,484,690 to de Giers, German Patent No. 2,758,379 and Italian Patent No. 619,958 to Carlo Ceresa et al.
An example of thick film resistor technology used in a liquid level sensor is shown in Weaver, U.S. Pat. No. 4,920,798. The Weaver device includes a thick film resistive coated plate with a slidable contact member providing a resistive signal in accordance with the position of the float.
A low cost liquid level sensor for use in fuel tank applications which is easily configured to the contours of the tank is needed. Further, a liquid level sensor for use in applications wherein the depth detection range of the sensor exceeds 50 centimeters is also needed.